Frozen food tray/cookware utilized in conventional ovens should have the capability of withstanding the great temperature variations existing between the temperature setting devices and the actual temperatures within the oven. Though the frozen food tray/cookware is only exposed to the oven's actual temperature, the user's expectations of the frozen food tray/cookware's capacity to withstand heat is a critical factor in the use of that frozen food tray/cookware. Putting frozen food tray/cookware that deforms at e.g. 200.degree. F. into an oven set for 325.degree. F. is clearly illogical. Equally illogical would be the use of the same frozen food tray/cookware in an oven whose temperature setting device fails to accurately control the oven's temperature. Thus a low temperature setting could result in a high oven temperature, and the frozen food tray/cookware would still deform. The realities of life are that few commercially available gas and electric ovens have accurate temperature controls and in most cases the ovens run hotter than the temperature setting. In a number of cases, an oven temperature setting of 400.degree. F. resulted in an oven temperature as high as 475.degree.-500.degree. F. This is the basis for the first sentence of this paragraph.
Plastics are typically termed thermoplastic or thermosetting. Thermoplastics are deformable with application of sufficient heat. Because thermosetting plastics (resins) are crosslinked, they are fairly resistant to heat deformation, certainly more so than thermoplastics. Consequently, thermosetting resins have been extensively used for the fabrication of cookware. For example, cookware has been made from melamine-formaldehyde resins, unsaturated polyester resins, and the like. Such plastics have excellent heat resistance. However, they do suffer from a number of significant deficiencies. Because they crosslink during their curing processes when molded, they shrink and pull away from the mold surfaces. Unless they are properly filled with small particulate fillers, the molded objects have very uneven surfaces, and they are subject to significant crazing and/or cracking. High filler loading adversely affects the physical properties of the molded object and precludes the direct obtaining of a glossy surface. Thermosetting resins are difficult to mold. They generally have to be compression or transfer molded. Such processes require much materials handling, large equipment, complicated and relatively expensive molds, and significant energy costs.
Thermoplastics have been used for coating paper dishware and some of them have been used as cookware. However, their use as cookware is severely restricted, generally to microwave oven applications. Thermoplastics, such as Udel.TM. polysulfone (made by Union Carbide Corporation), have been sold for use in making cookware designed for microwave oven applications. One would expect that some of such cookware has been generally employed in conventional ovens as well. However, Udel.TM. polysulfone has not proven to be suitable for the wide temperatures used in conventional oven cooking and hence, its usage in such applications has not been recommended.
Thermoformed crystallized polyethylene terephthalate (hereinafter "PET") is used for cookware in microwave and conventional oven units, but is generally limited in use to about 350.degree. F. Above this temperature, the modulus of the material drops rapidly so that cookware will sag and distort and will be unstable from a handling standpoint when removing from the oven with a food load present in the container. In the 400.degree. F. range, the polyethylene terephthalate containers will distort severely and lose their shape. If the PET is not crystallized during production of the cookware, it will only withstand a temperature of about 180.degree. F. before melting and, hence, is not at all suitable for conventional oven use.
Though the physical properties of a thermoplastic might,be considered at first blush to be the basis for its use as generally employable frozen food tray/cookware, i.e., frozen food tray/cookware usable in any kind of oven up to a temperature of 500.degree. F., such is clearly not the case. Since frozen food tray/cookware is in contact with the food placed therein, the plastic it is made from must be safe to use and not contaminate the food it contacts. Temperature gradients exist within conventional ovens and such a variable requires actual working information about a plastic's performance as frozen food tray/cookware under a wide variety of conditions.
In order to function as a frozen food tray as well as cookware, the plastic's performance must also be good at low temperature. For example, frozen food is commonly subjected to rough handling at temperatures as low as minus 40.degree. F. Since many plastics become brittle at these low temperatures, impact strength to withstand rough handling must be assured. As an illustrative example, the thermoformed crystallized PET discussed above is quite brittle at these low temperatures, and hence provides only marginally acceptable performance as a frozen food tray item.
Currently, frozen food trays are largely fabricated of aluminum or cardboard. An aluminum tray, although excellent for use in conventional ovens, cannot be microwaved without causing arcing in the microwave oven. In contrast, a cardboard tray can be microwaved but cannot withstand the high temperature of conventional ovens.
In view of the above, it would be highly desirable to provide a plastic frozen food tray/cookware characterized by impact resistance at low temperatures of about minus 40.degree. F., as well as the ability to withstand both microwave and conventional oven conditions with a minimum of sagging or distortion.